Definition,[u] is back-advanced, close (broad variant), rounded, short.
Articulation.The whole body of the tongue is in the back part of the mouth. -The part of the tongue nearer to the centre than to the back is raised just above the half-close position. There is no firm contact between the side rims of the tongue and the upper teeth. The tongue is rather lax. The lips are rounded about the same amount as for [o:] but the mouth Is not so open. The vowel is short.
This vowel may occur in closed stressed syllables. It never occurs in word initial positions, eg good, look, bosom, butcher, j
Allophones.The vowel [uj is longer in the closed syllable with a weak voiced consonant at the end. It is checked and much shorter in the stressed closed syllable ending in a strong voiceless consonant, cf pull — book.